A Low-Impact Eco House

Tucked away in a quiet cul-de-sac of Victorian terraces in the Roath district of Cardiff is a contemporary-style house so energy efficient and low-impact that it needs no central heating. In fact, it has no form of heating at all — for Paola Sassi’s new home was built to the incredibly tight German PassivHaus standard, meaning it is sustainable and low-energy, and only emits ultra-low levels of CO².

On a mid-January day, with an outside air temperature of just 7°C, it would not be unreasonable to expect a somewhat chilly welcome to a conspicuously unheated house. But far from it. When H&R visits, the internal temperature is a very comfortable 19°C. Lower than might be desired for a modern house but Paola’s attention to design detail – such as cladding internal walls in veneered plywood – ensure that the house has a warm feel.

“I needed somewhere to live and could have bought a house cheaper,” says Paola, “But I wanted to prove that it is possible to build a house in a city centre that is sustainable, comfortable and cheap to run.” And she has certainly achieved that. Paola is an architect teaching at the Cardiff University School of Architecture. She opted for PassivHaus as she was educated in Germany and understands the benefits of the renowned building standard (SEE BELOW).

Paola has gone to some trouble to ensure that the sun warms the house, with big triple-glazed windows to the south elevation and solar panels on the roof, both thermal and photovoltaic. These produce most of the hot water she requires for washing, showers, laundry etc, and about half the electricity she needs. There are dual-flush toilets, flow-regulated showers and taps, and a rainwater harvesting system which meets around half the water needs of the house.

The effect of all this is that the house costs less than £290 per annum to run and emits just 0.8 tonnes of CO² per year — both figures less than a quarter of the average for a house this size.

It was Paola’s time in practice in London that compelled her to focus on sustainable buildings. Working on large commercial projects, she became increasingly disenchanted with the levels of waste in the construction and use of the buildings. “It was our arrogance that we can do whatever we want that struck me,” says Paola. So she decided that her house would be built entirely from sustainable materials and that every part of it, from the floor covering to the zinc roof, would be reusable or recyclable.

As an example, hemp insulation was installed throughout. Hemp has very similar thermal properties to mineral wool, but has three significant advantages: it uses less energy in manufacture; it ‘locks in’ the CO² that the plant absorbed as it was growing; and it is entirely recyclable. When the house is finally demolished it can just be taken out and used again in another house — as can virtually every other component of the house, from the lime render to the timber cladding and the screws that hold them in place.

The internal walls are lined principally in sustainable plywood sheets, coated with a natural wax, which has the twin effect of allowing the ply to be completely recyclable and giving a warmer feel to the house.

A house with no heating needs a lot of insulation. Paola has installed 380mm in the walls, 200mm under the floor and 430mm in the roof. This compares to the normal UK standard of 90mm in the walls, 75mm under the floor and 270mm in the roof. It also needs a very high level of airtightness to prevent heat losses from air movement, and that is where Paola’s contractors encountered most of their problems.

Building airtight houses requires precision, and the typical British builder is not used to building houses in this way and to this standard. Paola’s contractors were forced to learn on the job, which led to a longer build time than expected, and to mistakes — with the extra cost involved in rectifying them.

Paola says: “It was very difficult to persuade people to build as I wanted them to. They wanted to do things their way.” The impact of this was that she did not always achieve what she wanted. For example, Paola’s is a timber frame house, all the timber being from sustainable forests. A typical UK timber frame will use 140mm timbers. Paola wanted a 200mm single-skin frame and the manufacturer given the contract decided he could not cope with this and pulled out. A second supplier then had to be found, and a way of manufacturing, delivering and erecting the frames agreed on. All of this added delays and cost to the project — so much so that Paola and her partner finally decided to take up tools and do the internal finishing work themselves.

“I knew going in that it would be tough, and really I took on too much,” she says, “but the house was worth it.” The project should have taken nine months but took 14. It should have come in at £1,200/m² but came in at £1,400/m². Expensive perhaps, but not as expensive as might be expected. The finished house is comfortable, inviting and worth at least what it cost to build. In the right market, it could even realise a profit.

Her advice to anyone contemplating a similar project is to stay hands-off. “I was architect, I sourced all the materials, selected and engaged the contractors, did the project management and even went on the tools. It was too much,” says Paola. “However, building a house like this is rewarding. It is sunny and warm; a comfortable living and working space. It costs very little to run — and I built it.”

What is Passivhaus?

PassivHaus was established by the PassivHaus Institut in Darmstadt, Germany, in 1996. Since then around 6,000 houses have been built and certified to the standard across Europe and the USA. What PassivHaus means, in broad terms, is that the building is insulated to a very high level (with a U-value that does not exceed 0.15W/m²/K) and that it allows the sun – through careful design – and other passive heat gains to produce enough energy to heat the home. Passive gain is the heat given off by people, cooking, the shower, even making toast or boiling the kettle. Pretty much everything we do produces heat which can be captured and circulated from warmer rooms (bathroom and kitchen) to cooler rooms (lounge and bedroom) by a heat recovery and ventilation system. Thus, when these measures are tied together, a PassivHaus does not need a traditional heating system or active cooling to be comfortable to live in.

In Europe, a dwelling is deemed to satisfy the PassivHaus criteria if:

• The total energy demand for space heating and cooling is less than 15kWh/m²/yr treated floor area. In a new UK house this figure is typically 55kWh/m²/yr
• The total primary energy use for all appliances, domestic hot water and space heating and cooling is less than 120kWh/m²/yr.

BRE is registered with the PassivHaus Institut Darmstadt to issue PassivHaus certification in the UK. See passivhaus.org.uk for more information.

Further reading:

• Eco House Design
• The Code for Sustainable Homes
• A European Green Home

Find out more about building an eco home in The Homebuilding & Renovating Book of Green Homes